The present invention relates to a telecentric image-forming optical system through which a principal ray passes parallel to an optical axis, and more particularly to a telecentric image-forming optical system capable of changing image-forming magnifications.
The telecentric image-forming optical system in accordance with the present invention can be utilized within a measuring projector and within an optical measuring instrument. It can also be utilized as an image-forming optical system within exposure apparatus such as a color scanner, a laser plotter and the like.
A lens system in which the secondary focal point of an objective lens having a positive focal length coincides with the primary focal point of an eyepiece having a positive focal length is known as a telescopic system. When an object is placed in the vicinity of the primary focal point of the object lens of the telescopic system, an inverted image is formed in the vicinity of the secondary focal point of the eyepiece to form a telecentric image-forming optical system through which a principal ray passes parallel to an optical axis.
In this case, the magnification of a real image with respect to the object is the ratio of the focal length of the eyepiece to the focal length of the objective lens, and the magnification is not varied when the object distance is varied. This is the characteristic feature of the foregoing lens system. Utilizing this feature, a telecentric image-forming optical system is frequently used as an optical system within a measuring instrument which requires an accurate measurement of a dimension as disclosed in Japanese Patent Laid-Open Publication (Kokai) No. 51-96336.
To take advantage of the characteristic feature of the telecentric image-forming optical system (i.e., that a principal ray passes therethrough parallel to an optical axis), the system may be utilized as a relay lens to reconstruct an image, formed by a lens system placed forward, as an object of the lens system placed backward. For example, U.S. patent application Ser. No. 33,582 (filed on Apr. 1, 1987) now U.S. Pat. No. 4,810,068 discloses art in which a two-step telecentric optical system is used within a laser exposure apparatus, and in particular, within a laser plotter.
The image formation magnification of such a telecentric image-forming optical system is kept constant even if an object distance is varied. The magnification of such a system is determined only by the combination of focal lengths of lenses used therewithin.
The above-described optical system presents a problem inasmuch as if the dimension of a lens or a lens group is not processed as designed for a lens or a lens group is erroneously arranged or adjusted during assembly, then the image-forming magnification of the system is different from that intended. It has been difficult to correct such an error. Further, it has been very difficult to vary the magnification of such a system by more than twofold without degrading the quality of a formed image below a certain level while keeping the distance between the object and the image unchanged and the principal ray traveling parallel to an optical axis. Conventionally, it has been possible to vary a magnification no more than twofold, for example up to 1.5 or 1.8 times (as disclosed in U.S. patent application Ser. No. 33,582). However, in a laser plotter, art work can be carried out very effectively only if the reproduction accuracy is variable by more than twofold (depending on the nature of the product). Therefore, the provision of a telecentric image-forming optical system capable of varying magnification more than twofold without degrading the quality of a formed image has been desired.